1. Field of the Invention
This invention generally relates to design of electron detectors. Specifically, this invention relates to design of tandem microchannel plate and solid state electron detector having a high linearity of the amplification coefficient over a wide dynamic range of the input current signal.
2. Description of the Related Art
In a conventional scanning beam apparatus, a specimen under inspection is irradiated with a particle beam called a primary beam. For example, the irradiating primary particle beam can be an electron beam. The interaction of the primary particle beam with the specimen causes the specimen to emit electrons with kinetic energies ranging between zero electron-volts (eV) and the kinetic energy of the particles in the primary beam.
The electrons emitted by the specimen are classified according to their initial kinetic energies. The first group of electrons, with kinetic energies of up to 50 eV is called secondary electrons, or secondaries. The secondary electrons emitted by the specimen typically carry information about the topographical structure of the specimen.
The interaction of the primary beam with the specimen also causes the emission of a second class of electrons, called backscattered electrons. The backscattered electrons have energies ranging from 50 eV and up to the kinetic energy of the particles (electrons) in the primary beam and carry information about the topographical structure and the material composition of the specimen.
The secondary and backscattered electrons emitted by the specimen are collected using an electron detector. It should be noted that most of the existing electron detectors are capable of detecting only electrons with kinetic energies included in a predetermined detection energy range. In addition, the detection efficiency (the ratio of the number of detected electrons to the total number of secondary and backscattered electrons emitted from the specimen) generally increases with the increase of the electron energy. Accordingly, in order to detect the secondary and backscattered electrons with higher efficiency, it is advantageous to increase their kinetic energies. Typically, this is accomplished by accelerating the electrons in the electric field of the scanning beam apparatus. The aforementioned accelerating electric field can be produced by biasing the surface of the specimen and the surface of the electron detector, such as to create a suitable electric potential difference therebetween.
The aforementioned electron detector collects the electrons emitted by the specimen and generates an output electrical signal representative of the cumulative charge of the collected electrons, multiplied by the amplification factor of the detector. The electric signal produced by the electron detector is used in creating an image of the specimen. Depending on the nature of the electrons used in imaging (secondary or backscattered), the created image is indicative of the topographic and/or the material structure of the specimen. After the image of the area of the specimen irradiated by the primary beam spot is created using the secondary and/or backscattered electrons, the specimen is moved with respect to the irradiating primary electron beam so that the scanning beam apparatus can produce an image of the next area. The specimen can be moved in a continuous or stepwise manner.
Unfortunately, when the secondary and backscattered electrons emitted by the specimen are detected by a detector, the xe2x80x9ctransit timexe2x80x9d between the arrival of the detected electron and the collection of the amplified signal at the other side of the detector can vary, sometimes substantially. A wide variation in the transit time results in the decrease of the frequency response of the detector. That subsequently decreases the scanning speed of the particle beam apparatus because the apparatus has to xe2x80x9cwaitxe2x80x9d for the signal emitted by the irradiated spot of the specimen to clear the detector, before it can move on to scan the next spot. Because the width of the transit time distribution is proportional to the average electron travel time through the detector, it is advantageous to minimize the electron travel time by accelerating electrons in an electric field and/or by reducing the travel distance of the electrons.
Accordingly, in order to minimize the spread of the detector transit time in a scanning beam apparatus, the detector of the secondary and backscattered electrons preferably has a low profile so that it does not add significantly to the overall length of the electron travel path.
One type of electron detector which is presently used for detecting electrons in microcolunms is a microchannel plate (MCP) electron detector. The microchannel plate detector comprises a thin plate, typically manufactured from an insulating material, such as glass. For example, the plate can be a few hundred microns thick. The plate of the microchannel plate detector contains a plurality of thin, typically round channels, which pass through the bulk of the plate and connect the opposite faces of the plate. The inside surfaces of these channels are coated with a material having a good secondary electron emission coefficient. A potential difference is applied between the two faces of the microchannel plate to create an accelerating electric field inside the channels.
A typical microchannel plate detector can be used to amplify the input current signal with a gain of up to tens of thousands. The amplification factor of the microchannel plate detector has such a high value for the following reasons. First, as well known to persons of skill in the art, when an electron strikes a working surface of the microchannel plate detector, it releases additional electrons, the number of which goes up with the kinetic energy of the striking electron, to about 1 keV. To maximize the number of the released electrons, the working surface of the microchannel plate detector is manufactured of, or coated with a material having a good secondary emission coefficient. It will also be appreciated by those of skill in the art that before striking the working surface of the microchannel plate, the secondary and backscattered electrons emitted by the specimen under examination are accelerated by the electric field of the microcolumn to a few hundred eV, or more.
Second, the electrons released from the working surfaces of the microchannel plate travel through the channels of the detector being accelerated in the electric field created by a potential difference applied to the opposite faces of the microchannel plate. During their travel inside the channels of the microchannel plate detector, the electrons strike the inside surfaces thereof, releasing greater and greater numbers of additional electrons. These additional electrons are also accelerated and strike the walls, which results in the production of even greater numbers of electrons. Accordingly, the described avalanche-like electron production results in exceptionally high signal amplification in the microchannel plate detector.
Because the number of the electrons released in each collision is related to the energy of the striking electron, the amplification factor of the microchannel plate detector depends on the potential of the front face of the detector with respect to the specimen, and the potential difference applied to the opposite faces of the detector.
The electronic current signal amplified by the microchannel plate is collected by a collector electrode and measured by a current monitor. A 100 pA input signal can give a 1 microamp output current at a gain of 10,000. Due to the aforementioned exceptionally high gain of the microchannel plate detector, detection of the input signals down to 1 pA is routinely possible. A major drawback of the microchannel plate detector is that its output current is substantially limited. When the output current of the detector exceeds about 10% of the microchannel plate strip current, the gain of the microchannel plate decreases causing the amplification factor of the device to be nonlinear. A typical strip current for the microchannel plate detector used in the described microcolumn is about 10 microamps. At a gain of 10,000, for a 1 microamp output current, the input current signal should not exceed 100 pA, for good linearity. To maintain linearity in the 10 nA input current signal range, the gain has to be below 100. Therefore, to maintain a good linearity and avoid saturation of the microchannel plate at high input electron currents, the gain of the microchannel plate has to be reduced so that the output signal of the microchannel plate does not exceed its upper linearity limit, or 10% of the microchannel plate strip current. Accordingly, the microchannel plate detector is not suitable for operating at high signal gains and high input current signals.
Accordingly, the dynamic range of the input electron current signal in the microchannel plate detector is substantially limited. As well known to persons of skill in the art, the term dynamic range of the microchannel plate refers to the ratio of the highest input electron current, which can be amplified by the microchannel plate in a linear manner to the lowest detectable electron current.
Accordingly, there exists a need for, at it would be advantageous to have a compact electron detector, which would offer a large bandwidth and high linearity of the amplification factor in a wide dynamic range of the input current signal. Such detector could be used as a detector for secondary and backscattered electrons in a scanning electron beam system for inspection, lithography, metrology, and other related applications.
It is one feature of the present invention to provide a compact detector for secondary and backscattered electrons in a scanning particle beam system, to provide high linearity of the amplification over a wide dynamic range of the input current signal. The compactness of the detector makes it ideally suited also for miniature electron beam columns such as microcolumns.
To achieve the above and other benefits and advantages of the present invention, there is provided a tandem electron detector for detecting secondary and backscattered electrons. The inventive tandem electron detector comprises a solid state detector and a microchannel plate detector.